Dual trailer arrangements particularly for gravel transport are known where there is a first trailer portion with a king pin at the front end for attachment to the fifth wheel of the tractor and two or three axles at the rear end. At the rear end there is a fifth wheel for receiving the king pin of a second trailer. In relation to gravel trailers it is common that the load box on the front trailer is located forward so that the load is applied primarily to the rear wheels of the tractor. The second trailer locates the load box in conventional symmetrical manner so as to apply its load to the rear wheels of the first trailer and the rear wheels of the second trailer. Commonly the front load box dumps to the side and the rear lead box dumps to the rear. Gravel trailers are typically empty on a return run. For reasons of economy it is desirable to lift ground wheels from the ground if they are not required to support a load. This reduces tire and wheel wear and reduces rolling resistance.
It is known to lift individual axles upwardly so that their wheels are removed from the roadway but this is complex and difficult to do on an automatic basis so that it is not widely used in gravel trucks.
U.S. Pat. No. 4,589,670 (Sweetin) issued May 20, 1986 shows an A-train arrangement in which a semi trailer is attached to the rear of a rigid truck by a trailing dolly and an arrangement for lifting the wheels of the dolly using a cylinder so that the trailer is supported temporarily only on the rear of the truck and the rear wheels of the trailer.
U.S. Pat. No. 2,126,819 (Schawlem) issued Aug. 16, 1938 shows a dolly towed behind a highway tractor with a trailer with a king pin mounted at the center of the dolly. The rear wheels of the dolly can then be lifted by a winch.
However both of these arrangements are used for back-up maneuverability and not for travel with the wheels raised at zero load.